Tips for running OVZ inside KVM effectively

So its come up a few times about running OVZ inside KVM.... @chief mentioned it previously a few times, most recently as an example way to 'bundle' your OS to make migrations between providers easier (in the context of those paying month to month).

The google summary seems to be: its doable and performance doesn't have to suck - but was hoping peeps that have experience doing this can share any tips.

In terms of efficient IO, is VT-D passthru possible thru all 3 layers?)

Assume a single IP for now (so port forwarding or similar network setup).

Comments

From a hosts point of view.. what is regular for great I/O (and this is for big servers, but I wouldn't do nested virtualization on anything smaller in production ) I think many use 8-12 disk arrays, SSD caching to improve I/O, customized OVZ kernels which I cannot give information on. That's all I can think of right now after just waking up relating to performance, but that's from a hosts point of view.

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@craigb I would say that for best I/O performance you should make sure that VirtIO is enabled and set the Linux I/O scheduler to "noop". You can do that by editing grub.cfg or menu.lst (depending on your distro) and add to the "kernel" line "elevator=noop" - and of course reboot. You can also switch the scheduler on the fly but I would say that this is safer. By doing this I/O should pass trough all the layers down to the host node.